Effect of microstructure on liquation cracking during AZ91 friction stir spot welding

Abstract

Effect of microstructures on liquation cracking is investigated. Melted eutectic formation and cracking are observed in the stir zone region of friction stir spot welds made in squeeze cast and in incompletely solutionised AZ91 sections containing Mg₁₇Al₁₂ particles. Crack susceptibility in the stir zone region is dramatically decreased in AZ91 base material which has been forged and solution treated, and is completely free of Mg₁₇Al₁₂ particles. It is suggested that rapid heating during the tool penetration stage in friction stir spot welding creates undissolved Mg₁₇Al₁₂ particles, which facilitate eutectic melting when the temperature in the stir zone reaches 437°C, the (α-Mg+Mg₁₇Al₁₂) eutectic temperature in the binary Mg–Al equilibrium phase diagram.